text: About 3% of human proteins are known to be "mode of action" drug targets, i.e., proteins through which approved drugs act. Another 7% of the human proteins interact with small molecule chemicals. Based on DrugCentral, 1795 human proteins annotated to interact with 2455 approved drugs. 
  Furthermore, it is estimated that only 10-15% of human proteins are disease modifying while only 10-15% are druggable (there is no correlation between the two), meaning that only between 1-2.25% of disease modifying proteins are likely to be druggable. Hence it appears that the number of new undiscovered drug targets is very limited.
  A potentially much larger percentage of proteins could be made druggable if protein–protein interactions could be disrupted by small molecules.  However the majority of these interactions occur between relatively flat surfaces of the interacting protein partners and it is very difficult for small molecules to bind with high affinity to these surfaces. Hence these types of binding sites on proteins are generally thought to be undruggable but there has been some progress (by 2009) targeting these sites.
  Chemoproteomics techniques have recently expanded the scope of what is deemed a druggable target through the identification of covalently modifiable sites across the proteome.